Foamed thermoplastic materials have become very popular as lightweight structural substitutes for metal in many applications, such as office equipment chassis and various automobile components. In general, the foams are obtained by incorporating a blowing agent into a mixture of the main body of a polymer such as polypropylene, polystyrene, polycarbonate, or polyphenylene ether (PPE). Upon exposure to elevated temperatures, the blowing agent decomposes to form gaseous decomposition products which expand the polymer into the article of interest. The resulting product is characterized by a much lower density as compared to a solid product, which in turn advantageously results in weight reduction and raw material savings.
Blowing agents which are suitable for foaming various thermoplastics are known in the art. Examples include hydrazodicarboxylates, benzamides, and amide derivatives of azodicarboxylic acid, as well as agents which are based on sodium bicarbonate or aluminum hydroxide. The agents are often used in the form of a concentrate, e.g., a blend of the agent itself with a polymer which acts as a carrier. The concentrate can also include plasticizers, e.g., a phosphate such as triphenyl phosphate; as well as deactivators such as zinc oxide.
While blowing agents such as the azodicarboxylic acid-derivatives are very effective in reducing the density of foamed articles, their use can occasionally result in several problems. For example, during the preparation of azodicarboxylic-based concentrates, excessive compounding or extrusion temperatures can destabilize the concentrate, leading to premature expansion of the carrier resin. This in turn can result in undesirable surface appearance, poor cell structure, and less weight reduction for a given amount of blowing agent concentrate.
If the melt viscosity of the carrier resin could be lowered, processing temperatures would not have to be as high, and the above-mentioned problems could be avoided. While the phosphate plasticizers mentioned above help to lower the melt viscosity, it is sometimes desirable not to have this type of plasticizer in the blowing agent concentrate. For example, the volatility of phosphate plasticizers can sometimes result in undesirable "plate-out" during injection molding processes.
Another plasticizer which helps to lower melt viscosity in these concentrates is mineral oil. However, such a material does not always become fully incorporated into the carrier resin matrix, especially when used at high levels. Thus, some of the mineral oil may migrate to the surface of foamed articles, leading to an undesirable appearance and possible paint adhesion problems.
In view of these remarks and the current state of the art, it's apparent that a need exists for improved blowing agent concentrates. The concentrates should have melt viscosities low enough to permit compounding and extrusion without any premature destabilization. Furthermore, the surface migration of any plasticizing agents in the concentrate should be minimized. Moreover, use of the concentrate should permit even and complete dispersion of the blowing agent itself during the expansion process, resulting in a foamed article having substantially uniform cell structure and good surface appearance. Finally, structural foam properties should be maintained.